To reduce the energy consumption per day in outdoor lighting system, light output is reduced or dimmed during non-occupancy or low utilization period such as in the middle of night. There are various methods in prior art to achieve dimming e.g. presence based dimming, time based dimming etc. These dimming methods are employed on individual lighting units or at group level. Most common dimming profile employed are time based dimming e.g. full brightness between dusk to 11 PM and 20-30% dimming from 11 PM to dawn. Dimming control is built in each lighting system in decentralized solar system. In prior art; in centralized system individual lighting units are communicated through wire or wireless i.e. PLC or zig bee or RF communication, but these are complex & costly. Additionally in some applications “time based dimming profile” is built in individual lamps. In these applications, it is very difficult to alter the dimming profile. Further, they need RTOS (real-time operating system) based system otherwise timer will vary from one lamp unit to another.
We also know that the overall system efficiency decreases significantly at lower dimming levels in AC grid due to decrease in efficiency of the inverter. Therefore, DC grid is becoming more popular. Though DC grid is better than AC grid, they also suffer efficiency loss when operated at lower dimming level due to decrease in efficiency of LED drivers. These result in higher PV and battery capacity and in turn results in higher system cost. Therefore, there is need of low cost efficient dimmable outdoor lighting system.
In the prior art, the lamps are usually operating at a constant power to deliver a constant lumen output despite the power source/battery state. If the battery is completely consumed, a black out occurs and the lighting system can not emit any light, which is very dangerous and inconvenient for the users. Thus it would be advantageous to have a more sophisticated lighting control mechanism that avoids black out.
U.S. Pat. No. 5,140,229A1 discloses that constant power control with the rated power is effected when the supply voltage is high and power lower than the rated power is supplied to the metal halide lamp in accordance with the reduction in power supply voltage.
EP1465395A2 discloses there is an up-converter that can be bypassed or switched on according to the output of the voltage source.